Problem: Kevin is 21 years older than Omar. Fifteen years ago, Kevin was 4 times as old as Omar. How old is Omar now?
We can use the given information to write down two equations that describe the ages of Kevin and Omar. Let Kevin's current age be $k$ and Omar's current age be $o$ The information in the first sentence can be expressed in the following equation: $k = o + 21$ Fifteen years ago, Kevin was $k - 15$ years old, and Omar was $o - 15$ years old. The information in the second sentence can be expressed in the following equation: $k - 15 = 4(o - 15)$ Now we have two independent equations, and we can solve for our two unknowns. Because we are looking for $o$ , it might be easiest to use our first equation for $k$ and substitute it into our second equation. Our first equation is: $k = o + 21$ . Substituting this into our second equation, we get the equation: $(o + 21)$ $-$ $15 = 4(o - 15)$ which combines the information about $o$ from both of our original equations. Simplifying both sides of this equation, we get: $o + 6 = 4 o - 60$ Solving for $o$ , we get: $3 o = 66$ $o = 22$.